The present invention relates to the mechanical structure of a pressurized battery or cell. More particularly, the present invention relates to an improved structure for a cell such as a nickel-hydrogen cell.
Metal gas cells such as nickel-hydrogen cells are known in the art. Such cells are contained in sealed vessels or casing which contain hydrogen gas under high pressure. Each cell has at least one nickel-containing positive electrode which is spaced from a hydrogen-forming negative electrode. The electrode generally are in the form of plates which are stacked as a plate stack. The stack also includes gas diffusion plates and separators which prevent short circuiting contact between the electrodes, and which hold a sufficient quantity of electrolyte for desired cell performance.
The electrolyte is typically an alkaline medium such as an aqueous solution of alkali metal hydroxide, generally an approximately 30 percent potassium hydroxide solution. The negative (hydrogen-forming) electrode is a plastic bonded, metal power plate. The metal powder is preferably one such as platinum or palladium which will catalyze a hydrogen dissociation reaction in the aqueous electrolyte. The plastic bonding material is desirably tetrafluoroethylene such as "Teflon" brand material made by duPont. The active material of the positive plate is generally a nickel-oxy-hydroxide.
The pressure vessel or casing is generally maintained at super-atmospheric pressure, for example a pressure in the range of 20 to 50 atmospheres. Hydrogen in the vessel diffuses through a diffusion mesh of Teflon or the like to reach the catalytic negative electrode or anode. The anode causes molecular H.sub.2 to dissociate into atomic hydrogen which in turn reacts with free hydroxyl groups to form water plus free electrons. The water and the free electrons react with the nickel-oxy-hydroxide positive to form nickel hydroxide plus free hydroxyl groups. Reverse reaction occur during charging. Ni-H.sub.2 cells currently in use have a cylindrical casing with domed ends, to withstand the internal gas pressure. Internally, the plate stack is supported and compressed by a nut or other stop on a retaining pin or rod which extends through a central or axial aperture in the stack. Axial terminals project outwardly from the centers of the domed ends of the casing. The terminals support the retaining rod and the cell stack in the casing. In another prior art construction the plates are compressed by a spring on a central rod on which they are stacked and which is again supported by axial terminals.
Ni-H.sub.2 batteries are quickly becoming the preferred electrical storage system for earth-orbiting satellites. The reasons for this are the long life of the Ni-H.sub.2 cell, its wide operating temperature range and most importantly, its high energy density.
Due to the expense of these satellites, the chance of cell failure must be absolutely minimized. The cells must be designed to be durable and withstand the forces encountered when the satellite is launched. Further, it is also critical that the mass and volume of these cells be as low as possible.
In such Ni-H.sub.2 cells the compression of the plate stack on a central rod on which they are stacked and which is in turn supported on axial terminals, leads to relatively great mass and volume of the battery, and places stress on the terminals. This makes such cells more prone to failure. Further, the axial location of the terminals increases the overall mass and volume of the system.